In recent years, in an optical network field applying a wavelength division multiplexing (WDM) technology, it is possible to configure an optical network having complex topology, such as a ring interconnection network or a mesh network, by realizing an optical add/drop multiplexing (OADM) apparatus which realizes the adding/dropping of optical signals without any opto-electronic conversion at a wavelength unit and the changing-over the optical signal path, or a wavelength cross-connecting (WXC) apparatus which is also called an optical hub.
In such an optical network, as one of factors determining the transmission quality of optical signal, there is the chromatic dispersion. In order to suppress the waveform degradation of optical signals by an influence of the chromatic dispersion, chromatic dispersion compensation modules are appropriately arranged along an optical transmission path to thereby perform the chromatic dispersion compensation, so that a residual chromatic dispersion value over the end-to-end of a path (to be referred to as a wavelength path hereunder) through which an optical signal of each wavelength transmitted over the optical network passes is converged in the dispersion tolerance at a terminal end (a reception end) of the wavelength path. Therefore, the design to determine a dispersion compensation amount of each chromatic dispersion compensation module plays an important role in deciding the transmission quality of the light signal.
Heretofore, as a chromatic dispersion compensation designing method for the ring interconnection network or the mesh network, there has been proposed a method of determining a dispersion compensation amount in accordance with a dispersion compensation map, in U.S. Pat. No. 6,580,861 and Japanese Laid-open Patent Publication No. 2004-274615. However, there are an error between a design value of the chromatic dispersion in an optical fiber used for an optical transmission path and an actual value thereof, and an error in the dispersion compensation which is caused by a generally discrete dispersion compensation amount in a chromatic dispersion compensation module. Since the accumulation of such errors is different for each wavelength path, it is problematically hard to realize the chromatic dispersion compensation corresponding to a desired dispersion compensation map on all of wavelength paths on the optical network.
As a conventional technology for solving the above problem, the invention in International Publication Pamphlet No. WO 2005/006604 discloses a designing method and a system of determining a dispersion compensation amount of a chromatic dispersion compensation module provided in each wavelength path, so that the residual chromatic dispersion at each terminal end point of a plurality of wavelength paths which is extracted from the optical network having the complex topology, is all within an allowable residual chromatic dispersion range set for all of wavelength paths. In such a conventional designing method, for each wavelength path, there is prepared the inequality provided that the sum of a chromatic dispersion value (constant) of an optical fiber along the wavelength path and the dispersion compensation amount (variable) of each chromatic dispersion compensation module arranged on each wavelength path is within a previously set allowable residual chromatic dispersion range (constant), and then, the simultaneous inequality for all of wavelength paths is solved to thereby judge whether or not there exists the solution of the dispersion compensation amount in each chromatic dispersion compensation module.
However, the following problems still remain in the conventional chromatic dispersion designing method as described above.
Firstly, in the conventional designing method, the solution of the dispersion compensation amount in each chromatic dispersion compensation module, which is obtained by the simultaneous inequality, is given within the required range. Therefore, if a plurality of combinations of dispersion compensation amounts in the respective chromatic dispersion compensation modules exists, there is a problem in that a specific solution method is not indicated as to which combination may be selected from these combinations.
Secondly, in the conventional designing method, there has been considered only the condition that the residual chromatic dispersion at each terminal end point of each wavelength path is within the previously set allowable residual chromatic dispersion range. Under such a condition, since the residual chromatic dispersion of each wavelength path along the way is not considered, there may be set the dispersion compensation amount resulting in the significant degradation of the optical signals due to the residual chromatic dispersion during the transmission. Consequently, even if the chromatic dispersion compensation in accordance with the design is performed at the terminal end point, there is a possibility that it becomes hard to restore the optical signal quality.